Simplify cookie usage in allowlist token serialization

We don't actually need to store the whole root notification, the token itself is enough.

Test: atest NotificationManagerServiceTest + manually
Bug: 320659371
Bug: 322326520
Bug: 322819634
Bug: 305695605
Change-Id: Ia5f54b309569fb807a8613fae2c3f95bd310232e